DesignCon 2022 Schedule Builder

Ever-increasing data rates demand more powerful transmitter feed-forward equalization (TxFFE): more taps and better simultaneous optimization of inter-symbol interference (ISI) correction and of noise. The complexity of tap optimization increases exponentially, each new tap introduces another dimension to the optimization equation. Multi-level modulation schemes like PAM4, PAM8, etc add yet another level of complexity; since tap granularity is limited by the transmitter's digital to analog converter (DAC) resolution, the more symbols, the fewer available tap values per symbol. We begin by reviewing techniques for tuning taps in practice, called link training, and for calibration of TxFFE signals for compliance and diagnostic tests. We then address three important issues: First, the best test points for calibrating TxFFE taps – the transmitter output, the test fixture output, or the input to a compliance board. Second, the most effective patterns for analyzing the properties of TxFFE signals; we'll show the pros and cons of a simple step response versus test patterns that produce ISI to make the TxFFE effect explicit, and the impact of finite DAC resolution for both. And, finally, we consider what optimized TxFFE taps can tell us about the channel, a question pivotal for understanding link training.

Takeaway

Clear and detailed understanding of TxFFE: optimization techniques, limitations imposed by the serdes' digital to analog conversion (DAC) resolution, the impact of noise and of the modulation scheme.

Intended Audience

General familiarity with high speed I/O and signal integrity